About 50 firefighters are battling a bushfire burning out of control near properties in Sydney’s north.

Cooler temperatures were hoped to bring some relief to fire crews in Australia a day after Sydney sweated through its hottest day ever.

But NSW Rural Fire Service (RFS) said a 200-hectare fire was burning at Lovett Bay in the Ku-ring-gai national park on the outskirts of the city.

An RFS spokesman told AAP this afternoon the fire was burning near properties but was not a direct threat to any of them.

He said there were no mandatory evacuations being carried out but campers at The Basin camping ground in the national park had been advised to leave.

They were being transported out by ferry because West Head Road was closed, he said.

The cause of the fire was unknown, the RFS said. Motorists were being advised to avoid the area.

On its website, the RFS said firefighters were working to establish containment lines. Smoke was covering Terrey Hills, the northern beaches and north shore.

SYDNEY STRUGGLES IN THE HEAT

The mercury topped 45.8 degrees Celsius at Sydney's Observatory Hill at 2.55pm (4.55pm NZT) yesterday, breaking the previous record set in 1939 by half a degree. The city's highest temperature was a scorching 46.5C, recorded in Penrith, while Camden, Richmond and Sydney Airport all reached 46.4C.

More than 220 people had been treated for heat exposure or fainting by late afternoon, the Ambulance Service of NSW said.

The heatwave also stranded thousands of commuters, with dozens of trains delayed as steel wires buckled and a hose used to run a key signalling system melted. On the central coast, the heat caused an overhead wire to buckle onto a train, trapping about 250 passengers for half an hour.

Serious fires raged across NSW and Victoria, where a man was found dead in a burning vehicle in Seaton, 200 kilometres east of Melbourne.

Firefighters said the conditions were ''as bad as it can get'' and feared it could take weeks to contain one fire, which has burnt more than 48,000 hectares in Gippsland.

Heyfield incident controller Bill Johnstone said the fire could continue for days or even weeks.

''We're still experiencing some dynamic fire behaviour. The conditions are deteriorating,'' Johnstone said.

''It's a very dangerous environment we're experiencing ... it's probably as bad as it can get.''

The conditions in NSW were also hellish, firefighters said.

''This has made for very difficult conditions and there are a lot of very active fires. There has not been the cloud cover we expected,'' said the commissioner of the NSW Rural Fire Service, Shane Fitzsimmons.

As temperatures cooled and the southerly approached, lightning strikes sparked multiple small fires across the state, adding further stress to the fire fighting effort.

Even as thousands of front-line personnel battled the flames, the nation's peak emergency body - the Australasian Fire and Emergency Service Authorities Council - lodged a Senate inquiry submission warning of worse to come.

The capacity of fire and emergency services to respond to major natural disasters will need to be increased if extreme weather events become more frequent and intense due to climate change, its submission to an inquiry into extreme weather events said.

The heatwave was moving north today, with northern New South Wales and Queensland going on fire alert.